Add default option to FieldValueFactor

Closes #1450

Author: gmarz

Diff for: src/Nest/DSL/Query/Functions/FieldValueFactorDescriptor.cs

@@ -38,14 +38,23 @@ public class FieldValueFactorDescriptor<T>
 		internal FieldValueFactorModifier? _Modifier { get; set; }
 
 		[JsonProperty("missing")]
-		internal double? _Missing { get; set; }
+		internal double? _Missing { get; set; }
+
+		[JsonProperty("default")]
+		internal object _Default { get; set; }
 
 		public FieldValueFactorDescriptor<T> Field(Expression<Func<T, object>> field)
 		{
 			this._Field = field;
 			return this;
 		}
 
+		public FieldValueFactorDescriptor<T> Default(object defaultValue)
+		{
+			this._Default = defaultValue;
+			return this;
+		}
+
 		public FieldValueFactorDescriptor<T> Factor(double factor)
 		{
 			this._Factor = factor;

Diff for: src/Tests/Nest.Tests.Unit/Search/Query/Singles/FunctionScoreQueryJson.cs

@@ -20,7 +20,13 @@ public void FunctionScoreQuery()
 							f => f.Linear(x => x.FloatValue, d => d.Scale("0.3")),
 							f => f.Exp(x => x.DoubleValue, d => d.Scale("0.5")),
 							f => f.BoostFactor(2.0),
-							f => f.FieldValueFactor(op => op.Field(ff => ff.DoubleValue).Factor(2.5).Modifier(FieldValueFactorModifier.SquareRoot).Missing(1.0))
+							f => f.FieldValueFactor(op => op
+								.Field(ff => ff.DoubleValue)
+								.Factor(2.5)
+								.Modifier(FieldValueFactorModifier.SquareRoot)
+								.Missing(1.0)
+								.Default(0.0)
+							)
 						)
 						.ScoreMode(FunctionScoreMode.Sum)
 						.BoostMode(FunctionBoostMode.Replace)
@@ -40,7 +46,7 @@ public void FunctionScoreQuery()
                             {linear: { floatValue : { scale: '0.3'}}},
                             {exp:    { doubleValue: { scale: '0.5'}}}, 
                             {boost_factor: 2.0 },
-							{field_value_factor: { field: 'doubleValue', factor: 2.5, modifier: 'sqrt', missing: 1.0}}
+							{field_value_factor: { field: 'doubleValue', factor: 2.5, modifier: 'sqrt', missing: 1.0, default: 0.0}}
                         ],				
 						query : { match_all : {} },
                         score_mode: 'sum',