On downgrade user required to pay more than plan costs

[b3y0urs3lf]

Tested locally using commit 5668ce0.

Steps to reproduce:

1. Initiate and pay for subscription.
2. create plan than costs less than default downgrade sum (atm it is set to 1000, so plan cost would be i.e. 500)

3. Initiate subscription for already existing api key and previously created plan

Actual result: user asked to pay more than actual plan costs

Expected result: user should not be asked more than actual plan costs.

[ristoalas]

Actually, to me this seems like correct behavior -- if we have specified a minimum payment then it has to be upheld. The only problem I can see is that when the user asks for the pricing list, they should already see at least the minimum amount for every plan. Do you disagree?