Using a variable as a default in a function definition causes inconsistent highlighting #278

Prerequisites

• [ X ] Put an X between the brackets on this line if you have done all of the following:
  • Reproduced the problem in Safe Mode: http://flight-manual.atom.io/hacking-atom/sections/debugging/#using-safe-mode
  • Followed all applicable steps in the debugging guide: http://flight-manual.atom.io/hacking-atom/sections/debugging/
  • Checked the FAQs on the message board for common solutions: https://discuss.atom.io/c/faq
  • Checked that your issue isn't already filed: https://github.com/issues?utf8=✓&q=is%3Aissue+user%3Aatom
  • Checked that there is not already an Atom package that provides the described functionality: https://atom.io/packages

Description

Using a variable as a default in a function definition causes inconsistent highlighting of variable name. Specifically, the first letter or the variable will not be highlighted.

Steps to Reproduce

1. Open new file
2. Set language to Python
3. Enter def test(test_var = variable):

Expected behavior: variable should be highlighted completely.

Actual behavior: ariable is highlighted while v is not.

Reproduces how often: 100%

Versions

Windows 10
C:\WINDOWS\system32>atom --version

Atom : 1.31.1
Electron: 2.0.7
Chrome : 61.0.3163.100
Node : 8.9.3

C:\WINDOWS\system32>apm --version
apm 2.1.1
npm 6.2.0
node 8.9.3 x64
atom 1.31.1
python 3.6.4
git 2.17.0
visual studio

Additional Information

None