Support for Array and Set attribute values (phlex-ruby#546)

This adds support for Array or Set attribute values like this:

```ruby
div(class: ["bg-red-500", "rounded"])
# or
div(class: Set["bg-red-500", "rounded"])
```

Array or Set attribute values will be treated like a token list, and its
members will be joined with a space. This works as long as all elements
are either strings or symbols.

Additionally, this change removes the implicit conversion of attribute
values to a string by calling `to_s` on anything that is not recognized.
Now attribute values that are not either a String, a Symbol, an Array, a
Set, or a Hash with values of one of these types, will raise an
ArgumentError.

lib/phlex/sgml.rb

@@ -406,8 +406,12 @@ def __build_attributes__(attributes, buffer:)
 							end
 						}, buffer: buffer
 					)
+				when Array
+					buffer << " " << name << '="' << ERB::Escape.html_escape(v.compact.join(" ")) << '"'
+				when Set
+					buffer << " " << name << '="' << ERB::Escape.html_escape(v.to_a.compact.join(" ")) << '"'
 				else
-					buffer << " " << name << '="' << ERB::Escape.html_escape(v.to_s) << '"'
+					raise ArgumentError, "Element attributes must be either a Boolean, a String, a Symbol, an Array of Strings or Symbols, or a Hash with values of one of these types"
 				end
 			end
 

test/phlex/view/attributes.rb

@@ -27,6 +27,66 @@ def template
 		end
 	end
 
+	with "an array of string attributes" do
+		view do
+			def template
+				div(class: %w(bg-red-500 rounded))
+			end
+		end
+
+		it "joins the array with a space" do
+			expect(output).to be == %(<div class="bg-red-500 rounded"></div>)
+		end
+	end
+
+	with "an array of symbol attributes" do
+		view do
+			def template
+				div(class: %i(bg-red-500 rounded))
+			end
+		end
+
+		it "joins the array with a space" do
+			expect(output).to be == %(<div class="bg-red-500 rounded"></div>)
+		end
+	end
+
+	with "an array of symbol and string attributes" do
+		view do
+			def template
+				div(class: ["bg-red-500", :rounded])
+			end
+		end
+
+		it "joins the array with a space" do
+			expect(output).to be == %(<div class="bg-red-500 rounded"></div>)
+		end
+	end
+
+	with "a set of string attributes" do
+		view do
+			def template
+				div(class: Set["bg-red-500", "rounded"])
+			end
+		end
+
+		it "joins the array with a space" do
+			expect(output).to be == %(<div class="bg-red-500 rounded"></div>)
+		end
+	end
+
+	with "an object that is not a boolean, String, Symbol, Array, or Hash" do
+		view do
+			def template
+				div(class: Object.new)
+			end
+		end
+
+		it "raises a Phlex::ArgumentError" do
+			expect { output }.to raise_exception(Phlex::ArgumentError)
+		end
+	end
+
 	if RUBY_ENGINE == "ruby"
 		with "unique tag attributes" do
 			view do