Handle downcased params

[josh-works]

I'm copy-pasting some of my comment from #19 here.

tl;dr I believe the email RFC is one where we should not make judgements on the capitalization (or lack there of) of email headers, like To, From, etc.

From https://tools.ietf.org/html/rfc2821#section-2.4:

Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command and extension name keywords) are not case sensitive, with the sole exception in this specification of a mailbox local-part (SMTP Extensions may explicitly specify case-sensitive elements)

The "mailbox local-part" would be smith in smith@company.com; RFC281 says that smith@email.com is distinct from Smith@email.com, but goes on to say

However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged.

Ah. RFC 822, section 3.4.7 CASE INDEPENDENCE:

Except as noted, alphabetic strings may be represented in any combination of upper and lower case.

When matching any other syntactic unit, case is to be ignored. For example, the field-names "From", "FROM", "from", and even "FroM" are semantically equal and should all be treated identically.

---

I, and others, like @zeknox were thrown for a bit when we tried to use standard Ruby hash syntax for testing griddler-mailgun in our applications.

we built hashes like this:

email = {
	to: 'arya@stark.com',
	from: 'jon@stark.com'
}

Because this hash does not matching the expected casing, griddler-mailgun was trying to operate on a nil, and not surfacing the actual problem to the user.

This PR updates the Griddler::Mailgun::Adapter class to downcase all params and headers; it of course does not change the output of the class; just updates some of the internals.

All tests still pass. I don't do OS PRs a lot, so I'm more than happy to make any modifications. If this PR is accepted, #27 should be rejected and closed, as that documentation update would now be made irrelevant.

I'm happy to squash this PR into a single commit, as well. Just let me know what would be helpful!