Make all of GsonHttpMessageConvertersConfiguration conditional on Gson

Previously GsonHttpMessageConvertersConfiguration was unconditional
with its nested configuration class being
@ConditionalOnClass(Gson.class). This led to the unnecessary
registration of the GsonHttpMessageConvertersConfiguration bean when
Gson isn't on the classpath.

This commit moves the condition up onto the outer class so that no
Gson-related beans are created when Gson is not on the classpath.

Closes gh-2778

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/GsonHttpMessageConvertersConfiguration.java

@@ -36,10 +36,10 @@ import com.google.gson.Gson;
  * @since 1.2.2
  */
 @Configuration
+@ConditionalOnClass(Gson.class)
 class GsonHttpMessageConvertersConfiguration {
 
 	@Configuration
-	@ConditionalOnClass(Gson.class)
 	@ConditionalOnBean(Gson.class)
 	@Conditional(PreferGsonOrMissingJacksonCondition.class)
 	protected static class GsonHttpMessageConverterConfiguration {