Set the quadding of a variable text field by default on creation

The quadding /Q of a field has a default value, so it seems that it is
not necessary to explicitly set the field to its default value of 0.

There are (at least) two reasons why it still makes sense to set it
explicitly:

* In certain circumstances [1] the default value for an unset /Q might
  be taken from somewhere else.

* When HexaPDF generates an appearance, it retrieves the value of the
  quadding via field.text_alignment which automatically sets /Q to its
  default value if it is not set. In case of adding a field to a PDF
  but *not* filling it (so no explicit /Q value), then signing the PDF,
  then filling in the field and again signing it, the initial signature
  will be invalided by the now explicitly set /Q.

[1]: #89 (comment)

CHANGELOG.md

@@ -8,7 +8,7 @@
 ### Changed
 
 * [HexaPDF::Type::AcroForm::Form] text-like field creation methods to always set
-  a default appearance string
+  a default appearance string and the quadding
 * Convenience methods for accessing resources to not add the deprecated /ProcSet
   entry by default
 * [HexaPDF::DigitalSignature::CMSHandler] to add informational output regarding

lib/hexapdf/type/acro_form/form.rb

@@ -558,7 +558,7 @@ def apply_variable_text_properties(field, font: nil, font_options: nil, font_siz
                                               font_options: font_options || {},
                                               font_size: font_size || 0,
                                               font_color: font_color || 0)
-          field.text_alignment(align) if align
+          field.text_alignment(align || :left)
         end
 
         def perform_validation # :nodoc:

test/hexapdf/type/acro_form/test_form.rb

@@ -164,6 +164,7 @@ def applies_variable_text_properties(method, **args)
       assert_equal(:'Helvetica', @acro_form.default_resources.font(font_name)[:BaseFont])
       assert_equal(0, font_size)
       assert_equal(HexaPDF::Content::ColorSpace::DeviceGray.new.color(0), font_color)
+      assert_equal(0, field.value[:Q])
 
       field = @acro_form.send(method, "field", **args, font: 'Times')
       font_name, font_size, font_color = field.parse_default_appearance_string