Small performance fix

[dshemin]

Move static regexp to a global variable in order to avoid multiple
regexp pattern compilations.

Before fix:

BenchmarkPopulateQueryParameters-8   	    5000	    303218 ns/op	  217132 B/op	    2323 allocs/op
BenchmarkTemplateToSwaggerPath/with_JSON_names-8         	  200000	      9134 ns/op	    5848 B/op	     130 allocs/op
BenchmarkTemplateToSwaggerPath/without_JSON_names-8      	  200000	      9467 ns/op	    5486 B/op	     134 allocs/op

After fix:

BenchmarkPopulateQueryParameters-8   	   20000	     80937 ns/op	   10088 B/op	     345 allocs/op
BenchmarkTemplateToSwaggerPath/with_JSON_names-8         	  200000	      5846 ns/op	    2860 B/op	      99 allocs/op
BenchmarkTemplateToSwaggerPath/without_JSON_names-8      	  200000	      6039 ns/op	    2486 B/op	     103 allocs/op

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[johanbrandhorst]

Great thinking, thanks for the contribution! Could you please sign the CLA?

[dshemin]

@googlebot I signed it!

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[dshemin]

@googlebot I fixed it.

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[johanbrandhorst]

Thanks for your contribution!