self.specified_fields not thread-safe

[danielstockton]

self.specified_fields can be shared across threads. This can produce strange results, when you issue multiple requests in quick succession with different specified fields. I don't think a resource should be modified after initialization, since it's initialized once per process.

[dan-klasson]

Your bug is definitely confirmed. To be honest I did not foresee this and I'm not really contributing to this code anymore. If you can provide a work around for this I would gladly appreciate it. If not, I am glad you brought it to my attention. And in absence of finding a work around, I will make a comment in the README.md file.