Validate domain in mg.Send()

Hi!

I just spent some time debugging error, invalid character 'M' looking for beginning of value, and realized my domain was incorrectly set. I later found this issue: #110

It is a silly mistake, but it may be helpful to return a clearer error message. What do you all think about validating the domain in addition to checking for an empty string? I could try to put in a PR if you all are interested. We could use a regular expression to test domain validity or maybe just check that the domain does not start with https?://?

mailgun-go/messages.go

Line 549 in 677f15d

if mg.domain == "" {

Thanks!

Here's more information if others run into this problem:

Error:

invalid character 'M' looking for beginning of value

Error originates from here:

mailgun-go/rest_shim.go

Line 81 in 677f15d

return response.parseFromJSON(v)

Cause:

Invalid API endpoint returns a plaintext response body, where there should be JSON

curl -i  --location --request POST 'https://api.mailgun.net/v3/http://mail.mydomain.com/messages' \
--header 'Authorization: Basic [API Key]' \
--form 'from="'\''Excited user mailgun@mail.mydomain.com'\
--form 'to="user@mail.org"' \
--form 'subject="'\''Hello'\
--form 'text="'\''Testing some Mailgun awesomeness!'

HTTP/1.1 200 OK

Mailgun Magnificent API