Possible typo/error in /src/display/annotation_layer.js with the "MouseUp" eventName

[Snuffleupagus]

In the following spots a "MouseUp" eventName is being used, which does look inconsistent considering that the "Mouse Down"/"Mouse Enter"/"Mouse Exit" eventNames all have a space in them.
Is this inconsistency actually expected, or is it a simple typo? If the former, then a comment explaining why would be appropriate; otherwise we should just fix this :-)

• pdf.js/src/display/annotation_layer.js

  Line 806 in 25b84ce

  ["mouseup", "MouseUp"],

• pdf.js/src/display/annotation_layer.js

  Line 945 in 25b84ce

  ["mouseup", "MouseUp"],

• pdf.js/src/display/annotation_layer.js

  Line 1034 in 25b84ce

  ["mouseup", "MouseUp"],

• pdf.js/src/display/annotation_layer.js

  Line 1168 in 25b84ce

  ["mouseup", "MouseUp"],

/cc @calixteman