Fix prediction heuristic (#5955)

* Relax check for prediction.
* Relax test in spark test.
* Add tests in C++.

demo/regression/machine.conf

@@ -7,24 +7,22 @@ objective = reg:squarederror
 
 # Tree Booster Parameters
 # step size shrinkage
-eta = 1.0 
+eta = 1.0
 # minimum loss reduction required to make a further partition
-gamma = 1.0 
+gamma = 1.0
 # minimum sum of instance weight(hessian) needed in a child
-min_child_weight = 1 
+min_child_weight = 1
 # maximum depth of a tree
-max_depth = 3 
+max_depth = 3
 
 # Task parameters
 # the number of round to do boosting
 num_round = 2
 # 0 means do not save any model except the final round model
-save_period = 0 
+save_period = 0
 # The path of training data
-data = "machine.txt.train" 
+data = "machine.txt.train"
 # The path of validation data, used to monitor training process, here [test] sets name of the validation set
-eval[test] = "machine.txt.test" 
-# The path of test data 
-test:data = "machine.txt.test"      
-
-
+eval[test] = "machine.txt.test"
+# The path of test data
+test:data = "machine.txt.test"