Fix gsub bug in deforce code

There is an edge case where namespaced fields starting with the char `c` are getting corrupted.

I've added a failing spec to explain the issue:
```ruby
Forcifier should deforce field names with __c correctly
     Failure/Error: fields.should == 'id,name,field1,ns__ctest'
       expected: "id,name,field1,ns__ctest"
            got: "id,name,field1,nstest" (using ==)
```

This PR fixes the issue.