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Added
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* Docs: Change log and contributing guide pages.
* set_text() method for the Text response.
* set_subtitle(), set_image() and set_buttons() methods for the Card response.
* set_title() and set_quick_replies() to the QuickReplies response.
Fixed
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* Fix missing fields in Card and QuickReply responses.
* Fix optional parameters for all rich responses.
* Fix parsing of Image and Card responses from requests.
* Fix RichResponse instantiation (shouldn't be able to instantiate an abstract
base class).
Improved
~~~~~~~~
* Docs: improve classes and methods docstrings.
Changed
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* Docs: Change theme to Read the Docs' theme.