contrast common hover label font color w/ bgcolor

... and fix tests that were asserting white on white
    hover label text.

Author: etpinard

src/components/fx/hover.js

@@ -703,6 +703,7 @@ function createHoverText(hoverData, opts, gd) {
 
         var commonBgColor = commonLabelOpts.bgcolor || Color.defaultLine;
         var commonStroke = commonLabelOpts.bordercolor || Color.contrast(commonBgColor);
+        var contrastColor = Color.contrast(commonBgColor);
 
         lpath.style({
             fill: commonBgColor,
@@ -713,7 +714,7 @@ function createHoverText(hoverData, opts, gd) {
             .call(Drawing.font,
                 commonLabelOpts.font.family || fontFamily,
                 commonLabelOpts.font.size || fontSize,
-                commonLabelOpts.font.color || Color.background
+                commonLabelOpts.font.color || contrastColor
              )
             .call(svgTextUtils.positionText, 0, 0)
             .call(svgTextUtils.convertToTspans, gd);

test/jasmine/tests/hover_label_test.js

@@ -2062,7 +2062,7 @@ describe('Test hover label custom styling:', function() {
             });
             assertCommonLabel({
                 path: ['rgb(255, 255, 255)', 'rgb(68, 68, 68)'],
-                text: [13, 'Arial', 'rgb(255, 255, 255)']
+                text: [13, 'Arial', 'rgb(68, 68, 68)']
             });
         })
         .then(function() {
@@ -2074,7 +2074,7 @@ describe('Test hover label custom styling:', function() {
             });
             assertCommonLabel({
                 path: ['rgb(255, 255, 255)', 'rgb(68, 68, 68)'],
-                text: [13, 'Arial', 'rgb(255, 255, 255)']
+                text: [13, 'Arial', 'rgb(68, 68, 68)']
             });
         })
         .then(function() {
@@ -2086,7 +2086,7 @@ describe('Test hover label custom styling:', function() {
             });
             assertCommonLabel({
                 path: ['rgb(255, 255, 255)', 'rgb(68, 68, 68)'],
-                text: [13, 'Arial', 'rgb(255, 255, 255)']
+                text: [13, 'Arial', 'rgb(68, 68, 68)']
             });
 
             // test arrayOk case
@@ -2113,7 +2113,7 @@ describe('Test hover label custom styling:', function() {
             assertPtLabel(null);
             assertCommonLabel({
                 path: ['rgb(255, 255, 255)', 'rgb(68, 68, 68)'],
-                text: [13, 'Arial', 'rgb(255, 255, 255)']
+                text: [13, 'Arial', 'rgb(68, 68, 68)']
             });
 
             // test base case