[Snackbar] Don't force uppercasing of the button.

Summary:
The snackbar button already handles the uppercasing. Remove the code in
Snackbar that uppercased the set title.

Closes https://github.com/google/material-components-ios/issues/618.

Test Plan:
Check the accessibility label of snackbar buttons in Catalog. It should not be
uppercased.

Reviewers: ajsecord, O1 Material components iOS, junius

Reviewed By: ajsecord, O1 Material components iOS, junius

Tags: #material_components_ios

Differential Revision: http://codereview.cc/D1123

components/Snackbar/src/private/MDCSnackbarMessageView.m

@@ -364,13 +364,11 @@ - (instancetype)initWithMessage:(MDCSnackbarMessage *)message
       [buttonView addSubview:button];
 
       // Style the text in the button.
-      NSString *uppercaseButtonText =
-          [message.action.title uppercaseStringWithLocale:[NSLocale currentLocale]];
       NSAttributedString *buttonText =
-          [[NSAttributedString alloc] initWithString:uppercaseButtonText
+          [[NSAttributedString alloc] initWithString:message.action.title
                                           attributes:buttonAttributes];
       NSAttributedString *buttonHighlightedText =
-          [[NSAttributedString alloc] initWithString:uppercaseButtonText
+          [[NSAttributedString alloc] initWithString:message.action.title
                                           attributes:buttonHighlightedAttributes];
       button.titleLabel.numberOfLines = 1;
       button.contentHorizontalAlignment = UIControlContentHorizontalAlignmentRight;