Be consistent with method names checking if a field is required

Across the repo the `<field-name>_required?` format is the most
common.

core/app/models/spree/order.rb

@@ -128,7 +128,7 @@ def states
     before_create :create_token
     before_create :link_by_email
 
-    validates :email, presence: true, if: :require_email
+    validates :email, presence: true, if: :email_required?
     validates :email, 'spree/email' => true, allow_blank: true
     validates :guest_token, presence: { allow_nil: true }
     validates :number, presence: true, uniqueness: { allow_blank: true, case_sensitive: true }
@@ -750,10 +750,15 @@ def link_by_email
     end
 
     # Determine if email is required (we don't want validation errors before we hit the checkout)
-    def require_email
+    def email_required?
       true unless new_record? || ['cart', 'address'].include?(state)
     end
 
+    def require_email
+      Spree::Deprecation.warn "Use email_required? instead", caller(1)
+      email_required?
+    end
+
     def ensure_inventory_units
       if has_checkout_step?("delivery")
         inventory_validator = Spree::Stock::InventoryValidator.new

core/spec/models/spree/order/validations_spec.rb

@@ -8,7 +8,7 @@ module Spree
       # Regression test for https://github.com/spree/spree/issues/2214
       it "does not return two error messages when email is blank" do
         order = Spree::Order.new
-        allow(order).to receive_messages(require_email: true)
+        allow(order).to receive_messages(email_required?: true)
         order.valid?
         expect(order.errors[:email]).to eq(["can't be blank"])
       end