Sort start and end override params

Fixes #375

lib/cartodb/models/dataview/histogram.js

@@ -174,8 +174,8 @@ Histogram.prototype.sql = function(psql, override, callback) {
         basicsQuery = overrideBasicsQueryTpl({
             _query: _query,
             _column: _column,
-            _start: override.start,
-            _end: override.end
+            _start: getBinStart(override),
+            _end: getBinEnd(override)
         });
 
         binsQuery = [
@@ -248,7 +248,7 @@ Histogram.prototype.format = function(result, override) {
         width = firstRow.bin_width || width;
         avg = firstRow.avg_val;
         nulls = firstRow.nulls_count;
-        binsStart = override.hasOwnProperty('start') ? override.start : firstRow.min;
+        binsStart = override.hasOwnProperty('start') ? getBinStart(override) : firstRow.min;
 
         buckets = result.rows.map(function(row) {
             return _.omit(row, 'bins_number', 'bin_width', 'nulls_count', 'avg_val');
@@ -266,9 +266,19 @@ Histogram.prototype.format = function(result, override) {
 };
 
 function getBinStart(override) {
+    if (override.hasOwnProperty('start') && override.hasOwnProperty('end')) {
+        return Math.min(override.start, override.end);
+    }
     return override.start || 0;
 }
 
+function getBinEnd(override) {
+    if (override.hasOwnProperty('start') && override.hasOwnProperty('end')) {
+        return Math.max(override.start, override.end);
+    }
+    return override.end || 0;
+}
+
 function getBinsCount(override) {
     return override.bins || 0;
 }