Handle (numeric) indexed access of DOMStringMap.

Source/core/dom/DatasetDOMStringMap.cpp

Index: Source/core/dom/DatasetDOMStringMap.cpp
diff --git a/Source/core/dom/DatasetDOMStringMap.cpp b/Source/core/dom/DatasetDOMStringMap.cpp
index 309dabfa323bdd13288032e050ae973d2a072ca1..82f9188bbd6f8446babde561da43497aa2690f45 100644
--- a/Source/core/dom/DatasetDOMStringMap.cpp
+++ b/Source/core/dom/DatasetDOMStringMap.cpp
@@ -83,7 +83,7 @@ static bool propertyNameMatchesAttributeName(const String& propertyName, const S
     unsigned p = 0;
     bool wordBoundary = false;
     while (a < attributeLength && p < propertyLength) {
-        if (attributeName[a] == '-' && a + 1 < attributeLength && attributeName[a + 1] != '-')
+        if (attributeName[a] == '-' && a + 1 < attributeLength && isASCIILower(attributeName[a + 1]))
             wordBoundary = true;
         else {
             if ((wordBoundary ? toASCIIUpper(attributeName[a]) : attributeName[a]) != propertyName[p])