Fix for issue 256: CodedOutputStream computes wrong size for negative enums

Negative enum values are encoded as sign-extended varints, but
CodedOutputStream.computeEnumSizeNoTag just returns
computeRawVarint32Size(). Use computeInt32SizeNoTag() instead
to match the writing code.

Adds a unittest for this in CodedOutputStreamTest.java

[jasonh, 2011-02-18 04:14:46]

Hi Jisi -

This fixes the bug when serializing negative enum values in Java, reported in
issue 256.

[Jisi Liu, 2011-02-18 04:29:19]

LGTM

Thanks for the fix!