[MRG+1] Incomplete submit button

[olafdietsche]

If there is no value attribute, provides the empty string as described
in http://www.w3.org/TR/html5/forms.html#attr-button-value:

The value attribute gives the element's value for the purposes
of form submission. The element's value is the value of the
element's value attribute, if there is one, or the empty string
otherwise.

[codecov-io]

Current coverage is 82.24%

Merging #1472 into master will not affect coverage as of ba52af1

@@            master   #1472   diff @@
======================================
  Files          158     158       
  Stmts         8225    8225       
  Branches      1140    1140       
  Methods          0       0       
======================================
  Hit           6765    6765       
  Partial        264     264       
  Missed        1196    1196       

Review entire Coverage Diff as of ba52af1

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