What is the liquid syntax for adding custom html attributes to forms and form fields? For example, I can generate an email field with a custom class with, e.g., 

{% email_field "email", class:"custom" %}

which for the donation form generates something similar to:

<input class="custom user-success" id="donation_email" name="donation[email]" type="email" value="">

I want to be able to add two custom html attributes, "required" and "pattern", and expected the following liquid to work:

{% email_field "email", class:"custom", required:"", pattern:"email" %}

Which outputs:

<input required="" class="custom user-success" id="donation_email" name="donation[email]" type="email" value="">

So that syntax seems to work for the required attribute, but not for the pattern attribute. Why? How do I add the pattern attribute to the output form field?