Fix misnamed variable in SwiftReader.

actual_md5 was being used instead of _actual_md5 only when assigning
the md5 object. This resulted in all checks and updates of the md5
to be skipped.

When fixed it revealed another bug that the md5 of the read content
is always checked even if there is no _expected_etag.

Added tests

Change-Id: Iaf1e21324ce592049d02cd5be123604b99833e86
Add unit tests for _encode_meta_headers

Also move it to a module level function.
Change-Id: I2da64876815619a6db7ff208061df2191767806c
Merge "Fix misnamed variable in SwiftReader."