Convert blogid to number for new sites

Since `sites/new` is an undocumented endpoint, I mistakenly expected
blogid to be a number and ditched the conversion in 19d76d217bff6edb39fbbf7ad845abb88d0d2ffa

Fixes #2047
Merge pull request #2048 from wordpress-mobile/issue/2047-crash-new-site

Convert blogid to number for new sites