Fix geocode() rate error for source = dsk

[phively]

This is a 2-line fix for issue #150 allowing address vectors with more than 2500 elements to be geocoded with source = 'dsk'. I ran devtools::check() and roxygen2::roxygenise(".") without any problems.

This method is different from the one in pull request #156 which appears to introduce a bug when the if statement on line 120 is evaluated (if source != 'google' then string s would not exist).

Thanks for the fantastic package!