MONGOID-5067: Fix Storable.add_field_expression() when adding a nested String operator equivalent to an existing nested Symbol operator

[dalton-braze]

Summary

Storable.add_field_expression() doesn't correctly handle adding a nested String operator that is equivalent to an existing nested Symbol operator. This PR includes both the fix and a test reproducing the bug in the old code.

This is the second piece of a Symbol-String bug manifesting in a chained Query.where().where(). The first piece is addressed in MONGOID-5066 and its accompanying PR.

Original Repro

query.where(:f => { :$in => [...] }).where(:f => { "$in" => [...] })

Test Output (No Changes)

% rake spec

  1) Mongoid::Criteria::Queryable::Storable#add_field_expression when a String equivalent of an existing Symbol operator is added connects the equivalent operators with $and
     Failure/Error:
       modified.selector.should == {
         '$and' => [
           {
             "foo"=> {
               "$in"=>[2, 3]
             }
           }
         ],
         "foo"=> {
           :$in=> [1, 2]
     
       expected: {"$and"=>[{"foo"=>{"$in"=>[2, 3]}}], "foo"=>{:$in=>[1, 2]}}
            got: {"foo"=>{:$in=>[1, 2], "$in"=>[2, 3]}} (using ==)
       Diff:
       @@ -1,3 +1,2 @@
       -"$and" => [{"foo"=>{"$in"=>[2, 3]}}],
       -"foo" => {:$in=>[1, 2]},
       +"foo" => {:$in=>[1, 2], "$in"=>[2, 3]},

XYZ examples, 1 failure

Test Output (Fix Implemented)

% rake spec

XYZ examples, 0 failures

[p-mongo]

Closing per https://jira.mongodb.org/browse/MONGOID-5067.