fix: add missing space before headerClass (#138)

[LeoVie]

What kind of change does this PR introduce? (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

#138

What is the new behavior?

This PR adds the missing space, as described in #138

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

If you are very, very strict, this is a breaking change for the few people, that used a string returning function for headerClass and that did not realize, that their class is not getting applied.

Other information:

[spike-rabbit]

Thanks a lot for your contribution.

Based on the current code quality around header-cell.component.ts I think it is fine to merge without tests, as there is currently not even a test file set up.

@LeoVie please note, that we are currently doing some larger clean ups, like introducing types everywhere. So it may takes a while till this is actually released.