Trim accept tokens as in spec.

[sandinmyjoints]

What: Trim whitespace around tokens from the input element accept attribute.

Why: The spec says accept is a set of comma-separated tokens, and to obtain the list of tokens from the attribute, the user agent must split the attribute value on commas. The algorithm for splitting says to "Strip leading and trailing ASCII whitespace from token." as step 2.

How: Call String.trim on each token.

Checklist:

• [na] Documentation
• [x] Tests
• [x] Ready to be merged

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[sandinmyjoints]

Ping for review.

[ph-fritsche]

resolved by #1064