1. HttpMediaDrmCallback.executeProvisionRequest needs to specify
an empty byte[], else we do a GET instead of a POST.
2. Content-Type should not be set when making the provision
request, since there's no body.
3. DataSource implementations must correctly handle a non-null
body with zero length. CronetDataSource was not handling this
case. DefaultHttpDataSource was, but made a code modification
to make it a little clearer. OkHttpDataSource seems to handle
the case correctly, and it doens't look like the code can be
made clearer.
Issue #1925
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136042641
