Fix "number.toFixed is not function" JS error.

Issue:
We got the following error in console log
"number.toFixed is not function"

Steps to reproduce:
1) Click on statistics tab.
1) Select any database node.
2) then select Databases collection node.
3) The above error will appear into console.

Reason:
I discussed the issue with @Murtaza and here we are explicitly
converting values at server side to string as JSON do not directly
support NaN & Infinity values. Because of which NumberFormatter receives
values of type string and breaks.

Solution:
Overrides the NumberFormatter to support NaN & Infinity values.
and we need to parse the values again in float at client side.

web/pgadmin/static/js/backgrid/backgrid.pgadmin.js

@@ -884,6 +884,26 @@
     })
   });
 
+  /*
+   * Override NumberFormatter to support NaN, Infinity values.
+   * On client side, JSON do not directly support NaN & Infinity,
+   * we explicitly converted it into string format at server side
+   * and we need to parse it again in float at client side.
+   */
+  _.extend(Backgrid.NumberFormatter.prototype, {
+    fromRaw: function (number, model) {
+      if (_.isNull(number) || _.isUndefined(number)) return '';
+
+      number = parseFloat(number).toFixed(~~this.decimals);
+
+      var parts = number.split('.');
+      var integerPart = parts[0];
+      var decimalPart = parts[1] ? (this.decimalSeparator || '.') + parts[1] : '';
+
+      return integerPart.replace(this.HUMANIZED_NUM_RE, '$1' + this.orderSeparator) + decimalPart;
+    }
+  });
+
   return Backgrid;
 
 }));