Fix should_remove_source_branch

What does this MR do and why?

should_remove_source_branch is an information from a Merge Request which has a meaningful value once the merge request is merged. It is used to check if the source branch should have been removed. Its value was not properly updated when a MR is merged.

When a Merger is merging a MR using the GUI should_remove_source_branch is defined: the option box is ticked or not.

When the API is used to merge a MR, if should_remove_source_branch is given as a parameter it takes over
General->Merge Requests->Merge options->Enable "Delete source branch" option by default

which is named force_remove_source_branch when looking at the MR information

After this MR is merged, looking at should_remove_source_branch and force_remove_source_branch for a merged MR will show if the source branch should have been removed according to this table.

should_remove_source_branch	force_remove_source_branch	source branch should have been removed
true	x	true
false	x	false
null	true	true
null	false	false

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