Update src/CsvHelper/CsvParser.cs

When the GetChar method detects the end of buffer after reading a '\r'
character, it does not reset fieldLength after appending fieldLength
characters to field.  As a result, the calling method will
incorrectly append another field length characters from the new
buffer to the current field.
Update src/CsvHelper.Tests/CsvParserTests.cs

Unit test EndBufferTest added.  Check that fields are read correctly
if a \r character occurs at the end of a buffer.
Cleanup.