Use diff thresholds for domain, top-level domain

[jdpopkin]

Use a higher threshold when checking for an exact match with an entire
domain. This is useful because otherwise we have a problem where
corrections can "chain" - test@yahooo.cmo leads to a suggestion of
test@yahooo.com, which leads to a suggestion of yahoo.com.

If the user was already corrected once, they shouldn't have to be
corrected again - especially since they're using the answer we just
gave them.

[weilu]

Why wouldn't you want test@yahooo.cmo to be corrected to test@yahoo.com, which is more correct than test@yahooo.com?

[jdpopkin]

With this fix, test@yahooo.cmo is now corrected to test@yahoo.com. Prior to this fix, test@yahooo.cmo would be corrected to test@yahooo.com, which would be corrected to test@yahoo.com. (Try it out on Kicksend's sign up page - http://kicksend.com/join - it's pretty weird.)

I guess the test doesn't make that as clear as it should be - it just tests that only one suggestion is made. I might change that.

[jdpopkin]

Updated with a somewhat clearer spec and variable name.

Mailcheck currently suggests test@yahooo.com for test@yahooo.cmo. Then it would suggest test@yahoo.com after you accepted the test@yahooo.com suggestion. This PR fixes that, making it suggest test@yahoo.com for test@yahooo.cmo.

[weilu]

Thanks!