Fix getindex(::Object{String}, ::Symbol) and vice versa

[hhaensel]

Currently haskey(::Object) is true for both String and Symbol, but getindex() doesn't get it.

julia> o = JSON.parse("""{"a": "a"}""")
julia> haskey(o, :a)
true
julia> o[:a]
ERROR: MethodError: no method matching find_node_by_key(::JSON.Object{String, Any}, ::Symbol)
The function `find_node_by_key` exists, but no method is defined for this combination of argument types.

This PR fixes that issue.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.05%. Comparing base (7ffcc9d) to head (fc4fede).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/object.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
- Coverage   90.18%   90.05%   -0.14%     
==========================================
  Files           7        7              
  Lines        1345     1347       +2     
==========================================
  Hits         1213     1213              
- Misses        132      134       +2     

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[quinnj]

Thanks!