Adding defaultHeaders dictionary to generated API classes. defaultHeaders will be merged with headerParams before sending the request.

fix for https://github.com/swagger-api/swagger-codegen/issues/638

modules/swagger-codegen/src/main/resources/objc/api-body.mustache

@@ -7,6 +7,9 @@
 {{/imports}}
 {{newline}}
 
+@interface {{classname}} ()
+    @property (readwrite, nonatomic, strong) NSMutableDictionary *defaultHeaders;
+@end
 
 @implementation {{classname}}
 static NSString * basePath = @"{{basePath}}";
@@ -34,18 +37,19 @@ static NSString * basePath = @"{{basePath}}";
 }
 
 -(void) addHeader:(NSString*)value forKey:(NSString*)key {
-    [[self apiClient] setHeaderValue:value forKey:key];
+    [self.defaultHeaders setValue:value forKey:key];
 }
 
 -(id) init {
     self = [super init];
+    self.defaultHeaders = [NSMutableDictionary dictionary];
     [self apiClient];
     return self;
 }
 
 -(void) setHeaderValue:(NSString*) value
            forKey:(NSString*)key {
-    [[self apiClient] setHeaderValue:value forKey:key];
+    [self.defaultHeaders setValue:value forKey:key];
 }
 
 -(unsigned long) requestQueueSize {
@@ -82,7 +86,8 @@ static NSString * basePath = @"{{basePath}}";
         {{^collectionFormat}}queryParams[@"{{baseName}}"] = {{paramName}};{{/collectionFormat}}
     }
     {{/queryParams}}
-    NSMutableDictionary* headerParams = [[NSMutableDictionary alloc] init];
+    NSMutableDictionary* headerParams = [NSMutableDictionary dictionaryWithDictionary:self.defaultHeaders];
+
     {{#headerParams}}if({{paramName}} != nil)
         headerParams[@"{{baseName}}"] = {{paramName}};
     {{/headerParams}}