Ensure that relationship structs aren't clobbered

- There was no thought put in if a relationship struct already existed
  when it went to be used a second time so it was being clobbered.
- Fix #39

templates/07_relationship_to_one_eager.tpl

@@ -22,11 +22,15 @@ func ({{$varNameSingular}}L) Load{{$txt.Function.Name}}(e boil.Executor, singula
 
 	args := make([]interface{}, count)
 	if singular {
-		object.R = &{{$varNameSingular}}R{}
+		if object.R == nil {
+			object.R = &{{$varNameSingular}}R{}
+		}
 		args[0] = object.{{$txt.LocalTable.ColumnNameGo}}
 	} else {
 		for i, obj := range slice {
-			obj.R = &{{$varNameSingular}}R{}
+			if obj.R == nil {
+				obj.R = &{{$varNameSingular}}R{}
+			}
 			args[i] = obj.{{$txt.LocalTable.ColumnNameGo}}
 		}
 	}

templates/08_relationship_one_to_one_eager.tpl

@@ -22,11 +22,15 @@ func ({{$varNameSingular}}L) Load{{$txt.Function.Name}}(e boil.Executor, singula
 
 	args := make([]interface{}, count)
 	if singular {
-		object.R = &{{$varNameSingular}}R{}
+		if object.R == nil {
+			object.R = &{{$varNameSingular}}R{}
+		}
 		args[0] = object.{{$txt.LocalTable.ColumnNameGo}}
 	} else {
 		for i, obj := range slice {
-			obj.R = &{{$varNameSingular}}R{}
+			if obj.R == nil {
+				obj.R = &{{$varNameSingular}}R{}
+			}
 			args[i] = obj.{{$txt.LocalTable.ColumnNameGo}}
 		}
 	}

templates/09_relationship_to_many_eager.tpl

@@ -23,11 +23,15 @@ func ({{$varNameSingular}}L) Load{{$txt.Function.Name}}(e boil.Executor, singula
 
 	args := make([]interface{}, count)
 	if singular {
-		object.R = &{{$varNameSingular}}R{}
+		if object.R == nil {
+			object.R = &{{$varNameSingular}}R{}
+		}
 		args[0] = object.{{.Column | titleCase}}
 	} else {
 		for i, obj := range slice {
-			obj.R = &{{$varNameSingular}}R{}
+			if obj.R == nil {
+				obj.R = &{{$varNameSingular}}R{}
+			}
 			args[i] = obj.{{.Column | titleCase}}
 		}
 	}