[AIRFLOW-2765] Set default mime_charset to UTF-8
[AIRFLOW-2765] Set default mime_charset to UTF-8

Closes #3627 from jeffkpayne/AIRFLOW-2765

wileeam/airflow,apache/airflow,spektom/incubator-airflow,mtagle/airflow,Fokko/incubator-airflow,Fokko/incubator-airflow,sekikn/incubator-airflow,cfei18/incubator-airflow,Fokko/incubator-airflow,malmiron/incubator-airflow,gtoonstra/airflow,gtoonstra/airflow,sekikn/incubator-airflow,mtagle/airflow,wileeam/airflow,danielvdende/incubator-airflow,wileeam/airflow,spektom/incubator-airflow,sekikn/incubator-airflow,DinoCow/airflow,criccomini/airflow,DinoCow/airflow,jgao54/airflow,gtoonstra/airflow,akosel/incubator-airflow,cfei18/incubator-airflow,edgarRd/incubator-airflow,artwr/airflow,mrkm4ntr/incubator-airflow,adamhaney/airflow,artwr/airflow,apache/airflow,apache/incubator-airflow,sid88in/incubator-airflow,nathanielvarona/airflow,sid88in/incubator-airflow,nathanielvarona/airflow,adamhaney/airflow,apache/airflow,mistercrunch/airflow,sid88in/incubator-airflow,wooga/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,jgao54/airflow,bolkedebruin/airflow,nathanielvarona/airflow,dhuang/incubator-airflow,artwr/airflow,apache/airflow,mrkm4ntr/incubator-airflow,airbnb/airflow,apache/airflow,r39132/airflow,nathanielvarona/airflow,akosel/incubator-airflow,andyxhadji/incubator-airflow,sid88in/incubator-airflow,edgarRd/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,fenglu-g/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,fenglu-g/incubator-airflow,cfei18/incubator-airflow,spektom/incubator-airflow,fenglu-g/incubator-airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,criccomini/airflow,andyxhadji/incubator-airflow,mistercrunch/airflow,mtagle/airflow,nathanielvarona/airflow,lyft/incubator-airflow,wileeam/airflow,malmiron/incubator-airflow,wooga/airflow,r39132/airflow,lyft/incubator-airflow,fenglu-g/incubator-airflow,jgao54/airflow,bolkedebruin/airflow,bolkedebruin/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,edgarRd/incubator-airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,apache/incubator-airflow,r39132/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,criccomini/airflow,andyxhadji/incubator-airflow,dhuang/incubator-airflow,artwr/airflow,wooga/airflow,andyxhadji/incubator-airflow,mrkm4ntr/incubator-airflow,airbnb/airflow,DinoCow/airflow,apache/airflow,DinoCow/airflow,airbnb/airflow,apache/incubator-airflow,akosel/incubator-airflow,bolkedebruin/airflow,akosel/incubator-airflow,malmiron/incubator-airflow,cfei18/incubator-airflow,adamhaney/airflow,cfei18/incubator-airflow,wooga/airflow,gtoonstra/airflow,Acehaidrey/incubator-airflow,mtagle/airflow,apache/incubator-airflow,malmiron/incubator-airflow,criccomini/airflow,edgarRd/incubator-airflow,adamhaney/airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,r39132/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,spektom/incubator-airflow,Fokko/incubator-airflow,danielvdende/incubator-airflow,jgao54/airflow