Fix for issue with poor text spacing in PDF

[dmserebr]

Found the issue with poor kerning when trying to convert EML to PDF on Windows with the latest tool release version. (On Linux, I did not see the issue.)
Here is an example of initial EML and here is an example of the resulting PDF.

After some search, I found that it turns to be a known issue in wkhtmltopdf. After upgrading to the recent release of wkhtmltopdf, I still could reproduce the issue. For Windows, the only working solution for that seems to be increasing conversion DPI for the tool (by default it is 96). As it should not have severe side effects, I suggest increasing conversion DPI in the command that calls wkhtmltopdf.
I have tested the fix both on Windows and Ubuntu.

[nickrussler]

@dmserebr Looks good 👍 Thank you for the contribution and the good PR!

[dmserebr]

You're welcome, and thanks for the good tool that helped me in my work!