Fixing bugs in UTF8Encoding's GetMaxByteCount and GetMaxCharCount.

The previous implementation of GetMaxByteCount has two bugs: it incorrectly took into account the byte order mark and it was taking the max of a byte count (3) and a character count (EncoderFallback.MaxCharCount). This could result in returning the max number of bytes required to convert a string that was less than the number bytes used when converting the string.

GetMaxCharCount did not previously take into account DecoderFallback.MaxCharCount or the fact that there may still be part of a surrogate pair left on the buffer from a previous call to convert.

I removed the #if NET_2_0 conditionals and NotWorking category since these methods now return results consistent with Microsoft's implementation. I also added two simple tests for custom EncoderFallback and DecoderFallback cases.

This commit is licensed under MIT/X11.