PATCH Support and parameter passing through body

* Enables support for [PATCH](https://plaid.com/docs#Docs_Update_Credentials)
* Sends all parameters through the body as JSON
* Removes client side parameter validation - leaves it to the Plaid API
* Sends in MFA Code as string in test - *should probably be allowed as string or number in Plaid API*

Similar in concept to https://github.com/plaid/plaid-node/pull/9 - without the stylistic changes which were addressed in https://github.com/plaid/plaid-node/pull/7