Normative: handle awkward rounding behavior (#471)

Handle awkward rounding behavior when dealing with currencies and the
value of "maximumFractionDigits" is less than 2.

Fixes #239

spec/annexes.html

@@ -178,5 +178,8 @@ <h1>Additions and Changes That Introduce Incompatibilities with Prior Editions</
     <li>
       <emu-xref href="#sec-Intl-toStringTag"></emu-xref> In ECMA-402, 7th Edition, the @@toStringTag property of `Intl` was not defined. In 8th Edition, @@toStringTag is set to *"Intl"*.
     </li>
+    <li>
+      <emu-xref href="#sec-intl-numberformat-constructor"></emu-xref> In ECMA-402, 8th Edition, the NumberFormat constructor used to throw an error when style="currency" and maximumFractionDigits was set to a value lower than the default fractional digits for that currency. This behavior was corrected in the 9th edition, and it no longer throws an error.
+    </li>
   </ul>
 </emu-annex>

spec/numberformat.html

@@ -30,9 +30,12 @@ <h1>SetNumberFormatDigitOptions ( _intlObj_, _options_, _mnfdDefault_, _mxfdDefa
           1. Set _intlObj_.[[MaximumSignificantDigits]] to _mxsd_.
         1. Else if _mnfd_ is not *undefined* or _mxfd_ is not *undefined*, then
           1. Set _intlObj_.[[RoundingType]] to ~fractionDigits~.
-          1. Let _mnfd_ be ? DefaultNumberOption(_mnfd_, 0, 20, _mnfdDefault_).
-          1. Let _mxfdActualDefault_ be max( _mnfd_, _mxfdDefault_ ).
-          1. Let _mxfd_ be ? DefaultNumberOption(_mxfd_, _mnfd_, 20, _mxfdActualDefault_).
+          1. Let _specifiedMnfd_ be ? DefaultNumberOption(_mnfd_, 0, 20, *undefined*).
+          1. Let _specifiedMxfd_ be ? DefaultNumberOption(_mxfd_, 0, 20, *undefined*).
+          1. If _specifiedMxfd_ is not *undefined*, set _mnfdDefault_ to min(_mnfdDefault_, _specifiedMxfd_).
+          1. Set _mnfd_ to ! DefaultNumberOption(_specifiedMnfd_, 0, 20, _mnfdDefault_).
+          1. Set _mxfd_ to ! DefaultNumberOption(_specifiedMxfd_, 0, 20, max(_mxfdDefault_, _mnfd_)).
+          1. If _mnfd_ is greater than _mxfd_, throw a *RangeError* exception.
           1. Set _intlObj_.[[MinimumFractionDigits]] to _mnfd_.
           1. Set _intlObj_.[[MaximumFractionDigits]] to _mxfd_.
         1. Else if _notation_ is *"compact"*, then