Adding cornercases for set_metadata

Aggregate_set_metadata leaves users confused because it currently
does not check for duplicate key=value parameters for addition
or a missing key for deletion. All that user sees is that "metadata has been updated"
more details in bug1292572

This patch checks for above mentioned corner cases and informs users when and why
there was no change made in above corner cases

Adding better testcases also.

   Closes-Bug: #1292572

Change-Id: I820d793ac44680295df8b2a3f58a0834f7019875
Merge "Adding cornercases for set_metadata"