PUP-1724 Don't modify the paramaters to deep_merge

Instead of modifying the first paramater of deep_merge due to the
use of the merge! function, instead use merge to return a copy of
the merged object. This allows one to continue to use the original
first parameter after the call to deep_merge.
Merge pull request #220 from mediatemple/fix/master/deep_merge_fix

PUP-1724 Don't modify the paramaters to deep_merge