Bug with oid implicit cast inside C++

[tbranyen]

Found a segfault when using this.commit.sha() inside a Note.remove test. I switched over to this.commit.id() instead and the segfault disappeared. The error I received was:

*** Error in `/usr/bin/node': free(): invalid size: 0x00007fffdbe986c0 ***
======= Backtrace: =========
/usr/lib/libc.so.6(+0x7198e)[0x7f802fc2198e]
/usr/lib/libc.so.6(+0x76dee)[0x7f802fc26dee]
/usr/lib/libc.so.6(+0x775cb)[0x7f802fc275cb]
/home/tim/git/nodegit/nodegit/build/Release/nodegit.node(_ZN7GitNote6RemoveERKN2v820FunctionCallbackInfoINS0_5ValueEEE+0xb42)[0x7f802ccde772]
/usr/bin/node(_ZN2v88internal25FunctionCallbackArguments4CallEPFvRKNS_20FunctionCallbackInfoINS_5ValueEEEE+0xa7)[0x5f8ab7]
/usr/bin/node[0x61590c]
[0x22e51d20740e]

[johnhaley81]

Fixed in #531