fix encoding + symbol (#415)

* add + symbol as unreserved

* unit test for + symbol

Author: dejan2k

Example/Tests/Classes/LPOpenUrlMessageTemplateTest.m

@@ -26,7 +26,7 @@ - (void)tearDown {
 }
 
 - (void)testThatURLEncodingWorksForUnreservedCharacters {
-    //Test characters  @":-._~/?&=#"
+    //Test characters  @":-._~/?&=#+"
     // Note: all of them includes : . / charachers
 
     NSString *url;
@@ -74,6 +74,12 @@ - (void)testThatURLEncodingWorksForUnreservedCharacters {
     template = [self createTemplateWithUrl:url];
     encodedURL= [template urlEncodedStringFromString:[template.context stringNamed:LPMT_ARG_URL]];
     XCTAssertEqualObjects(url, encodedURL);
+    
+    //test for + sign
+    url = @"http://test.com/test+end";
+    template = [self createTemplateWithUrl:url];
+    encodedURL= [template urlEncodedStringFromString:[template.context stringNamed:LPMT_ARG_URL]];
+    XCTAssertEqualObjects(url, encodedURL);
 }
 
 - (LPOpenUrlMessageTemplate *)createTemplateWithUrl:(NSString *)url {

Leanplum-SDK/Classes/MessageTemplates/LPOpenUrlMessageTemplate.m

@@ -50,7 +50,7 @@ - (void) openURL
 }
 
 - (NSString *)urlEncodedStringFromString:(NSString *)urlString {
-    NSString *unreserved = @":-._~/?&=#";
+    NSString *unreserved = @":-._~/?&=#+";
     NSMutableCharacterSet *allowed = [NSMutableCharacterSet
                                       alphanumericCharacterSet];
     [allowed addCharactersInString:unreserved];