subplot() bugfix

[cpsievert]

Fixes a bug likely introduced by d4e2143.

The test shows where the bug is relevant

[chriddyp]

💃

[cpsievert]

The message below was automatically generated after build https://travis-ci.org/ropensci/plotly/builds/77173975

On TravisCI, commit 4bb006e was successfully merged with 12c51d2 (master) to create 9531e51. A visual testing table comparing 12c51d2 with 9531e51 can be found here ->

http://cpsievert.github.io/plotly-test-table/R/9531e51/

Detected 1 differences ->

http://cpsievert.github.io/plotly-test-table/R/9531e51//plotly-subplot-group

[cpsievert]

The message below was automatically generated after build https://travis-ci.org/ropensci/plotly/builds/77176619

On TravisCI, commit 4bb006e was successfully merged with 12c51d2 (master) to create f5e4b6f. A visual testing table comparing 12c51d2 with f5e4b6f can be found here ->

http://cpsievert.github.io/plotly-test-table/R/f5e4b6f/

Detected 1 differences ->

http://cpsievert.github.io/plotly-test-table/R/f5e4b6f//plotly-subplot-group