Seek by offset before write for fuse

[u8sand]

Writes after seeks on the fuse mounted filesystem fail, this fixes it and things like pd.HDFStore operability on these mounted filesystems.

[isidentical]

If you can, it would be really nice if we can add a test for that so future regressions won't happen though can see that this is a clear bug.

[u8sand]

@isidentical I wrote the test but discovered another bug.. the seek+read isn't working on the memory provider (added a commented out assertion) which is odd since BytesIO seems to do seeking just fine.

Update 1
Seeking works in both MemoryFileSystem and LocalFileSystem when in mode r+ but not when in mode w+ for either. While + in mode enables reading and writing at the same time, that would be a PR for another day.

Update 2
Nonetheless my simple patch fixes seek + writing which is necessary for certain kinds of writers (like h5 files). My tests were cleaned up accordingly to focus on this functionality and not read/write (+) mode.

[isidentical]

LGTM