Bug 812924 : Default Buffering + Unit Test to prove that, given bad defa...

...ult, we buffer to 128K  This rerolls brads original default buffering patch , confirming that in the case where users dont set their own io buffer size,  (i.e. they inherit the default 4096 byte buffer), we override it with the optimal buffer size which we deem to be 128 KB.  Obviously, one benefit of this test is that it additionally confirms that the shim buffers properly, and that our extension of the RawLocalFileSystem is configurable to support arbitrary write buffer size.