zapier convert: Fix session auth environment variable name

[eliangcs]

Addresses PDE-71.

Fixes two issues:

• When converting session auth, .env had user-facing auth fields like USERNAME and PASSWORD while it should have SESSION_KEY. (Because zapier test doesn't actually test session auth refreshing.)
• Use _.snakeCase(key).toUpperCase() instead of key.toUpperCase() to turn an auth field key to the environment variable name, so before the generated test code has this:

authData: {
  sessionKey: process.env.SESSIONKEY
}

Now it becomes:

authData: {
  sessionKey: process.env.SESSION_KEY
}

[BrunoBernardino]

Clean and simple! I didn't pull it down because our test coverage is very good here.

Good job!

Q: Could you link the JIRA card in the number next time? Makes it quicker for me to get there and get the whole context. #recommendation

[eliangcs]

@BrunoBernardino will make sure to do that next time. Thanks for the review and suggestion!