Fixed INPUT_FIELD_DEFAULT_MISMATCH (#104)

* Fixed `INPUT_FIELD_DEFAULT_MISMATCH`

* Adds example

* Fixed indentation

* Fix format

spec/Section 4 -- Composition.md

@@ -2491,16 +2491,17 @@ type User {
   - Let {defaultValues} be a set containing the default values of each input
     field in {inputFields}.
   - If the size of {defaultValues} is greater than 1:
-    - {InputFieldsHaveConsistentDefaults(inputFields)} must be false.
+    - {InputFieldsHaveConsistentDefaults(inputFields)} must be {true}.
 
 InputFieldsHaveConsistentDefaults(inputFields):
 
 - Given each pair of input fields {inputFieldA} and {inputFieldB} in
   {inputFields}:
-  - If the default value of {inputFieldA} is not equal to the default value of
-    {inputFieldB}:
-    - return false
-- return true
+  - If {inputFieldA} has a default value and {inputFieldB} has a default value:
+    - If the default value of {inputFieldA} is not equal to the default value of
+      {inputFieldB}:
+      - return {false}
+- return {true}
 
 **Explanatory Text**
 
@@ -2513,7 +2514,7 @@ different source schemas will result in a schema composition error.
 
 **Examples**
 
-In the the following example both source schemas have an input field `field1`
+In the the following example both source schemas have an input field `genre`
 with the same default value. This is valid:
 
 ```graphql example
@@ -2539,6 +2540,32 @@ enum Genre {
 }
 ```
 
+If only one of the source schemas defines a default value for a given input
+field, the composition is still valid:
+
+```graphql example
+# Schema A
+
+input BookFilter {
+  genre: Genre
+}
+
+enum Genre {
+  FANTASY
+  SCIENCE_FICTION
+}
+
+# Schema B
+input BookFilter {
+  genre: Genre = FANTASY
+}
+
+enum Genre {
+  FANTASY
+  SCIENCE_FICTION
+}
+```
+
 In the following example both source schemas define an input field
 `minPageCount` with different default values. This is invalid: