Single is_password prompt changes all further prompts

Hey there,

when using a prompt session the default value for is_password will be overwritten by a single call to prompt("Hello World", is_password=True). Any further prompts without having the attribute is_password set will still be in password mode.

Here's an example:

from prompt_toolkit import PromptSession
session = PromptSession()
session.prompt("Your Password:", is_password=True)
session.prompt("Your Magic Number:")

The output is the following:

Your Password:**** 
Your Magic Number:** 

Currently I have to explicitly set is_password=False on every further prompt after using a single prompt with is_password=True. Is this the expected behaviour? I would expect the second prompt to show the entered characters.
Our shouldn't you use a prompt session for entering a password? This however will makes things complicated when using a global style in your prompt session for example.

I'm using prompt_toolkit 3.0.0 from source at commit 850a4cdd21713060d5cbd1dfd923b69b1c696f4b

Start by reproducing the two-call PromptSession example shown in the issue, focusing on how prompt() handles the is_password argument between calls. Trace the PromptSession prompt entry point and add a regression test showing that a one-off password prompt does not affect the next prompt. Done means the second prompt displays entered characters without explicitly setting is_password=False.