set http = "post" for gnr_resolve queries longer than 300 records; 

fixes gh-262

Tests fail due to error that existed in previous commit (d91e2e0885eaeaaa064f8961749bb8fbc2ae4066).

```
tax_name : ................
tax_rank : ........
tnrs : Error in lapply(x, f) : object 'callopts' not found
Error in names(tmp) <- tolower(names(tmp)) :
  attempt to set an attribute on NULL
```