Allow for access token to be passed as either a string or symbol in GraphBatchAPI#graph_call_in_batch.

Introduces a simple symbolize_hash method in Koala::Utils to accomplish this.

Fixes #409
Merge pull request #419 from mattmueller/chore/allow_for_strings_and_symbols_in_batch_options_hash

GraphBatchAPI#graph_call_in_batch: allow for string or symbol options.