use Encoding.GetEndocing("ASCII") instead of Endocing.UTF8

src/ImageSharp/MetaData/Profiles/ICC/IccDataReader.cs

@@ -17,6 +17,7 @@ namespace ImageSharp
     internal sealed class IccDataReader
     {
         private static readonly bool IsLittleEndian = BitConverter.IsLittleEndian;
+        private static readonly Encoding AsciiEncoding = Encoding.GetEncoding("ASCII");
 
         /// <summary>
         /// The data that is read
@@ -129,8 +130,7 @@ namespace ImageSharp
         /// <returns>The value as a string</returns>
         public string ReadASCIIString(int length)
         {
-            // Encoding.ASCII is missing in netstandard1.1, use UTF8 instead because it's compatible with ASCII
-            string value = Encoding.UTF8.GetString(this.data, this.AddIndex(length), length);
+            string value = AsciiEncoding.GetString(this.data, this.AddIndex(length), length);
 
             // remove data after (potential) null terminator
             int pos = value.IndexOf('\0');

src/ImageSharp/MetaData/Profiles/ICC/IccDataWriter.cs

@@ -16,6 +16,7 @@ namespace ImageSharp
     internal sealed class IccDataWriter
     {
         private static readonly bool IsLittleEndian = BitConverter.IsLittleEndian;
+        private static readonly Encoding AsciiEncoding = Encoding.GetEncoding("ASCII");
 
         private static readonly double[,] IdentityMatrix = { { 1, 0, 0 }, { 0, 1, 0 }, { 0, 0, 1 } };
 
@@ -222,8 +223,7 @@ namespace ImageSharp
         /// <returns>the number of bytes written</returns>
         public int WriteASCIIString(string value)
         {
-            // Encoding.ASCII is missing in netstandard1.1, use UTF8 instead because it's compatible with ASCII
-            byte[] data = Encoding.UTF8.GetBytes(value);
+            byte[] data = AsciiEncoding.GetBytes(value);
             this.dataStream.Write(data, 0, data.Length);
             return data.Length;
         }
@@ -239,8 +239,7 @@ namespace ImageSharp
         {
             value = value.Substring(0, Math.Min(length - 1, value.Length));
 
-            // Encoding.ASCII is missing in netstandard1.1, use UTF8 instead because it's compatible with ASCII
-            byte[] textData = Encoding.UTF8.GetBytes(value);
+            byte[] textData = AsciiEncoding.GetBytes(value);
             int actualLength = Math.Min(length - 1, textData.Length);
             this.dataStream.Write(textData, 0, actualLength);
             for (int i = 0; i < length - actualLength; i++)

src/ImageSharp/MetaData/Profiles/ICC/TagDataEntries/IccDataTagDataEntry.cs

@@ -14,6 +14,8 @@ namespace ImageSharp
     /// </summary>
     internal sealed class IccDataTagDataEntry : IccTagDataEntry
     {
+        private static readonly Encoding AsciiEncoding = Encoding.GetEncoding("ASCII");
+
         /// <summary>
         /// Initializes a new instance of the <see cref="IccDataTagDataEntry"/> class.
         /// </summary>
@@ -67,8 +69,7 @@ namespace ImageSharp
             {
                 if (this.IsAscii)
                 {
-                    // Encoding.ASCII is missing in netstandard1.1, use UTF8 instead because it's compatible with ASCII
-                    return Encoding.UTF8.GetString(this.Data, 0, this.Data.Length);
+                    return AsciiEncoding.GetString(this.Data, 0, this.Data.Length);
                 }
                 else
                 {