allow negative values in numeric quiz question
allow negative values in numeric quiz question

fixes QO-376

test plan:
 - Add a numeric question to a quiz
 - change the question answer type to "Answer with Precision"
 - Change Precision to a value between 1 and 15
 - Type a negative answer into the answer field (or a zero)
 - The negative value should be formatted with the correct
   precision on blur (it should not change to NaN)
 - Students should be able to answer the question correctly

Change-Id: Iabf33557ca4544febe121c74df12837e5a73ed06
Reviewed-on: https://gerrit.instructure.com/156252
Tested-by: Jenkins
Reviewed-by: Mark Grant <8fea02099c20cab3b3cdc078b54afcf514bf5fcb@instructure.com>
QA-Review: Nathan Gbedemah <c6981ff3dcbdb6d4312c73db7f7dbc8832ed2063@instructure.com>
Product-Review: Kevin Dougherty <d976013e3ceb631f24c072156980cd9ccba4cf84@instructure.com>

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