Don't throw exceptions from ajax-form

[rnicholus]

From webcomponents.org's best practices page:

Failing silently is golden: Components should act like native DOM elements, so avoid creating elements that throw JS errors from ordinary DOM interactions, for example: you can place a <div> inside a <ul>, and while it may not behave or render normally, it shouldn't throw errors either.

This makes sense, and ajax-form should rely on default values or fail silently. One example relates to handling an invalid or unsupported method. Currently, an Error is thrown, but perhaps it should just revert to "GET" instead.

[rnicholus]

Completed and will be part of 2.0.