Fix fetching of greater than pagesize pubs

[jefferis]

Hi @jkeirstead,

Thanks for the nice package. I've noticed that fetching of >pagesize publications by get_publications seems to be broken (on develop and on master). For example for myself:

nrow(get_publications("cuXoCA8AAAAJ"))
nrow(get_publications('cuXoCA8AAAAJ', pagesize = 50))

both give 20 pubs. Proposed fix in attached.

I also suggest increasing pagesize since this has no substantial penalty for short publication lists but has a significant penalty for long lists. For example:

# a colleague with 187 pubs
system.time(get_publications('G6ufCyYAAAAJ', pagesize = 100))
## restart R session or otherwise clear cache
system.time(get_publications('G6ufCyYAAAAJ', pagesize = 20))

## -> 1.06 vs 3.74 seconds for me

[jkeirstead]

Lovely - thanks!

[jefferis]

Hi @jkeirstead. Thanks for merging this PR. I was wondering if you would consider merging develop into master or even pushing up to CRAN at some point. Thanks, Greg.