extract_tables does not extract properly when there are highlighted cells

[dsobo]

Please specify whether your issue is about:

If I try to run extract_tables on a PDF that has a table with highlighted cells then only the highlighted cells will end up in the final table. I am not quite sure how to do a reproducible example (i tried based on the how to) but I attached the PDF that does not work with extract_tables().

When I run extract_text on this same PDF all cell values end up in the resulting string.

• [x ] a possible bug
• a question about package functionality
• a suggested code or documentation change, improvement to the code, or feature request

If you are reporting (1) a bug or (2) a question about code, please supply:

• ensure that you can install and successfully load rJava
• a fully reproducible example using a publicly available dataset (or provide your data)
• if an error is occurring, include the output of traceback() run immediately after the error occurs
• the output of sessionInfo()

Put your code here:

## rJava loads successfully
# install.packages("rJava")
library("rJava")

## load package
library("tabulizer")
[pdf_test2.pdf](https://github.com/ropensci/tabulizer/files/3795083/pdf_test2.pdf)

## code goes here
tabulizer::extract_tables("pdf_test2.pdf")

## session info for your system
sessionInfo()
![sessionInfo](https://user-images.githubusercontent.com/37000302/67970975-f21d6b00-fbd9-11e9-9647-a1aca39f0e6b.png)