Complete issue#150 - Port BlobSroteWriter bug fix

- Port fix for issue couchbase-lite-java-core#96 — BlobStoreWriter bug
fix
- Add BlobStoreWriterTest.TestBlobStoreWriterForBody()
- Add Assertions to check replication’s LastError in several places
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Resolved merge conflicts.
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Merge branch 'issue-150'

Conflicts resolved:
	src/Couchbase.Lite.Shared/Replication.cs
	src/Couchbase.Lite.Shared/Replication/Pusher.cs
	src/Couchbase.Lite.Tests.Shared/ReplicationTest.cs