Test Encoder.Convert and UTF8Encoder.Convert behaviors.
Added class ExposedEncoder to test the virtual method Encoder.Convert behavior.
Test UTF8Encoder.Convert override.
Added classes EncodingEncoder and EncodingDecoder.
EncodingEncoder and EncodingDecoder override Encoder.Convert.
The Convert overrides allow retrieval of chars/bytes buffers with the same size as .NET.
Merge pull request #1190 from esdrubal/encoder-convert

System.Text.Encoder convert