Fix to prevent errors occuring when merging non-map value

[itoooo]

In situation overriding non-map value to a map, the merge-with function throws an exception like below:

(proton.core/deep-merge {:foo 1} {:foo {:bar :buzz}})
Execution error (IllegalArgumentException) at proton.core$deep_merge$m__4759/doInvoke (core.cljc:196).
contains? not supported on type: java.lang.Long

I assume the expected behavior is returning {:foo {:bar :buzz}} right?

In this PR, the deep-merge fn gets fix to pass only map values to the merge-with fn.
I also confirmed the tests passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.10%. Comparing base (6133961) to head (8ceb204).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   94.40%   95.10%   +0.69%     
==========================================
  Files           9        9              
  Lines         286      286              
  Branches        4        3       -1     
==========================================
+ Hits          270      272       +2     
+ Misses         12       11       -1     
+ Partials        4        3       -1     

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[federkasten]

LGTM 🙆‍♂️

[federkasten]

Successfully passed the tests by rebasing to #21. Thank you for your review! @itoooo