[SPARK-8940] [SPARKR] Don't overwrite given schema in createDataFrame

JIRA: https://issues.apache.org/jira/browse/SPARK-8940

The given `schema` parameter will be overwritten in `createDataFrame` now. If it is not null, we shouldn't overwrite it.

Author: Liang-Chi Hsieh <viirya@appier.com>

Closes #7311 from viirya/df_not_overwrite_schema and squashes the following commits:

2385139 [Liang-Chi Hsieh] Don't overwrite given schema if it is not null.

R/pkg/R/SQLContext.R

@@ -86,7 +86,9 @@ infer_type <- function(x) {
 createDataFrame <- function(sqlContext, data, schema = NULL, samplingRatio = 1.0) {
   if (is.data.frame(data)) {
       # get the names of columns, they will be put into RDD
-      schema <- names(data)
+      if (is.null(schema)) {
+        schema <- names(data)
+      }
       n <- nrow(data)
       m <- ncol(data)
       # get rid of factor type