PHP 7.4 Array and string offset access syntax with curly braces is deprecated

[roko91]

First thank you for the great package!

When testing PHP 7.4.0beta4 I encountered the following exception:

In HmacSha1.php line 15:

During class fetch: Uncaught ErrorException: Array and string offset access syntax with curly braces is deprecated in /home/test/test/vendor/abraham/twitteroauth/src/SignatureMethod.php:61
Stack trace:
#0 /home/test/test/vendor/composer/ClassLoader.php(444): Illuminate\Foundation\Bootstrap\HandleExceptions->handleError()
#1 /home/test/test/vendor/composer/ClassLoader.php(444): include()
#2 /home/test/test/vendor/composer/ClassLoader.php(322): Composer\Autoload\includeFile()
#3 [internal function]: Composer\Autoload\ClassLoader->loadClass()
#4 /home/test/test/vendor/abraham/twitteroauth/src/HmacSha1.php(15): spl_autoload_call()
#5 /home/test/test/vendor/composer/ClassLoader.php(444): include('/home/test/tes...')
#6 /home/test/test/vendor/composer/ClassLoader.php(322): Composer\Autoload\includeFile()
#7

I think it's pretty self-explanatory - for array and string offset access curly braces should be replaced with square braces for future PHP versions.

I will create PR for this tomorrow.

[roko91]

Never mind I just saw someone else did already create PR: #788